1. Field of the Invention
The present invention generally relates to a controller driver and a display panel driving method. More specifically, the present invention is directed to a technique of reducing power consumption of a controller driver used to drive a display panel.
2. Description of the Related Art
In general, a display panel such as a liquid crystal display (LCD) panel is driven by a controller driver. The controller driver is sometimes provided separately from the display panel, and the controller driver is sometimes provided with the display panel by using the COG (chip on glass) technique. The controller driver receives display data to be displayed, and stores the received display data in a display memory. Based on the display data stored in the display memory, the controller driver drives data lines of the display panel.
In the controller driver mounted on a mobile terminal such as a portable telephone and a PDA (personal data assistants), the reduction of power consumption is important. If the power consumption of the mobile terminal is reduced, the operation time during which the mobile terminal can be operated through a single recharging operation can be prolonged. The prolongation of such an operation time is effective to use the mobile terminal in more convenient manner.
Japanese Laid Open Patent Application (JP-P2002-182627A) discloses a technique of reducing the power consumption of a controller driver. The controller driver in this conventional example is provided with a latch circuit for receiving display data, and a display memory which does not contain a sense amplifier. A plurality of bits of the display data are latched by the latch circuit. The bit lines of the display memory are directly driven by the latch circuit. Since the sense amplifier is not used, the power consumption of this controller driver is low. Further, since the plurality of data bits are written in the display memory by the latch circuit, a total number of times when word lines of the display memory are activated is decreased. In other words, a total access time to the display memory is reduced, so that the power consumption of this controller driver is reduced.
One of factors which cause increase of power consumption in the controller driver is the increase in a data quantity of display data sent to the controller driver. Since various sorts of information are displayed on the display panel, display data sent to the controller driver increases rapidly. In some case, bit map data having high gradation are sent to the controller driver so as to display a photograph on a display panel. Also, in some case, video image data whose data quantity are large are sent to the controller driver in order to display the video image data on the display panel. In addition, in some case, bit map data of a character image are sent to the controller driver in order to display the character image on the display panels. However, every time a data bit of the display data is received, the controller driver consumes power to some extent. Therefore, the increase of the data quantity of display data directly causes increase of power consumption by the controller driver.
The increase of the data quantity of display data sent to the controller driver is also not preferable from the viewpoint of EMI (electromagnetic interference). However, undesirable radiation of electromagnetic wave from the controller driver cannot be avoided every time the data bit of the display data is received. As a consequence, the reduction of the data quantity of display data is important in suppression of the EMI.
A synthetic image that a background image and a character image are synthesized is one of the reasons why the display data sent to the controller driver increases. In a general controller driver, in order to display the synthetic image newly after a certain image is displayed, for example, for an on-screen display, an entire image to be displayed needs be newly resent to the controller driver.
A controller driver having a function to partially rewrite a displayed image can solve such a problem to some extent. The reason is in that the controller driver can display the synthetic image if only character bit map data to be synthesized is sent to the controller driver.
There is stronger requirement to decrease the data quantity of display data sent to the controller driver. Under such a requirement, provisions of techniques are demanded that the data quantity of display data to be sent to the controller driver having the function for synthesizing the background image and the character image can be further reduced.